1. Field of the Invention
The present invention relates to a method of producing liquid crystalline polymer films, particularly such as are self-supporting and anisotropic, which films are based on photocrosslinked liquid crystalline poly(meth)acrylates. These films can be used for the storage of optical data.
2. Description of the Background
Liquid crystalline polymers have been receiving great attention, particularly in the area of information storage. (See Allen, G., and Bevington, J., 1989, "Comprehensive Polymer Science", Vol. 5, pub. Pergamon Press, pp. 701-732.)
Various methods are known for orienting anisotropic thin liquid crystalline polymer films so that the films can be converted into liquid crystalline monodomains. The means of bringing about the orientation include, e.g.:
Electromagnetic fields (see Eur. OSs 0 231, 856; 0, 231, 857; and 0, 231, 858); PA1 Surface effects (Ger. Pat. Appl P 38 25 066); and PA1 Mechanical deformation (see Finkelmann, H., and Hammerschmidt, K., Makromol.Chem., 190, 1089-1101, 1989).
Macroscopic orientation takes place generally in a temperature range between the glass temperature (Tg) and the clearing temperature (Tn,i) of the polymer. In the case of liquid crystalline side chain polymers, the temperature is close to the clearing temperature.
Experience with the electromagnetic field and surface effect methods shows that for liquid crystalline polymer films one is always confined to a structured support material and to specified layer thicknesses. Thus, according to Ger. Pat. App. P 38 25 066 layers 1-2 micron thick can be oriented without problems. However, these methods come up against their limits when the layer is in the range of 10-20 microns thick. In the past, such layers have been prepared by display technology, which unfortunately involves stringent thermal stressing of the polymers. The mechanical deformation method (shearing, stretching, or compressing) is used for orientation of liquid crystalline main chain polymers and crosslinked liquid crystalline polymers (see Zentel, R., Finkelmann, H., et al., "Adv. Polym. Sci.", 60/61; M. Gordon, (Ed.), pub. Springer-Verlag, Heidelberg, pp. 155-162; Zentel, R., et al., 1987 Makromol. Chem., 188, 665-674; and Finkelmann, H., et al., 1987, Mol. Cryst. Liq. Cryst., 142, 85-100.)
The orientation of liquid crystalline elastomers is carried out analogously to that of linear side chain polymers below T=Tn,i (transition from the nematic to the isotropic phase) in the rubber-elastic state, and is frozen-in below the glass temperature. Liquid crystalline elastomers have a number of disadvantages. For example, crosslinked polysiloxanes have very low Tg (&lt;room temperature (RT)), so that the orientation only survives when stress is applied or at low temperatures (below Tg).
In the case of known crosslinked liquid crystalline poly(meth)acrylates produced by chemical crosslinking, Tg&gt;RT but film preparation is costly, and the prepared films are inadequately homogeneous. Thus there remains a need for devising suitable crosslinkable liquid crystalline polymers which can be converted to a macroscopically oriented anisotropic state by mechanical deformation over a practicably wide range of layer thicknesses, e.g. 5-50 micron, without the use of a support material.